Solder alloy, electronic board using the solder alloy, and method of manufacturing the electronic board

ABSTRACT

A solder alloy for flow soldering is disclosed that includes 3.0 wt % to 14.0 wt % Zn, 0.003 wt % to 0.05 wt % Al, and the balance of Sn.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2005/016580, filed on Sep. 9, 2005, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Sn—Zn—Al solder alloy, an electronic board using the same, and a method of manufacturing the electronic board.

2. Description of the Related Art

Conventionally, Sn—Pb-based alloys, which have low melting points and good solder wettability, are frequently used for solder joining in electric and electronic apparatuses. However, since Pb is poisonous, there is a strong demand for development of a lead-free solder alloy containing no Pb.

As lead-free alloys, alloys having Sn, Ag, Cu, Bi, Zn, In, etc., added in combination have been put into practical use so far, but are used for only special purposes with the exception of Sn—Ag—Cu-based solder. The Sn—Ag—Cu-based solder has a high melting point of 218° C. Therefore, electronic components and an electronic board should be heated to a temperature several tens of ° C. higher than the melting point in order to have solder joining performed evenly and with high reliability on the joints to be soldered (solder joints) of the electronic component and the electronic board. On the other hand, the solder joining should be performed under the condition of an extremely small margin because the heat resistance temperatures of some electronic components are approximately 240° C. Thus, the Sn—Ag—Cu-based solder, which puts electronic components under a great thermal stress, has many problems in the heat resistance of electronic components, in the reliability of their solder joining, and further in the reliability of solder-joined electronic apparatuses.

Therefore, Sn—Zn solder alloy (whose melting point is 199° C.) is proposed as solder paste approximately 20° C. lower in melting point than the Sn—Ag—Cu-based solder. However, in order to ensure good solderability, the Sn—Zn-based solder requires solder joining in a non-oxidizing atmosphere such as nitrogen gas because of its poor wettability due to fast oxidation of Zn.

A Sn—Zn—Al-based solder alloy is proposed as a solder alloy that enables solder joining in the atmosphere at a low melting point. (See Japanese Patent No. 3357045.) The Sn—Zn—Al-based alloy ensures good solder wettability by suppressing oxidation of the Sn—Zn alloy with the addition of Al.

Flow soldering, which has good mass productivity and is inexpensive, is the mainstream method in the solder joining of electronic components and electronic boards. In flow soldering, a flow of molten solder is formed, and soldering is performed by dipping an electronic board in the flow. According to flow soldering, molten solder is constantly circulated. Therefore, the distribution of solder temperatures is narrow, and the solder joining time per piece is reduced. This results in excellent mass productivity.

On the other hand, compared with dipping, which does not circulate molten solder, flow soldering, which circulates molten solder, has a problem in that an oxide is likely to be formed. In particular, in the case of using the Sn—Zn solder alloy, there is the problem of reduced solderability because Zn, which is easily oxidizable and less likely to be broken, causes a large amount of oxide scum, so-called dross, to be generated on the surface of molten solder so as to prevent molten non-oxidized solder from coming into direct contact with solder joints.

Usually, a flow soldering apparatus employs stainless steel, which is preferable in melting point and corrosion resistance, in its parts that come into contact with molten solder, such as a soldering bath and a pump. In the case of using lead-free solder such as Sn—Ag—Cu-based solder or Sn—Zn-based solder with the flow soldering apparatus, there is the problem of occurrence of solder erosion. The solder erosion is a phenomenon where the Sn component of solder erodes a bath wall, an impeller shaft, and components provided in a nozzle vessel, and causes the problems of solder leakage, reduction of apparatus capability, and occurrence of failure.

SUMMARY OF THE INVENTION

Embodiments of the present invention may solve or reduce one or more of the above-described problems.

According to one embodiment of the present invention, there are provided a solder alloy used for flow soldering in which one or more of the above-described problems may be solved or reduced, an electronic board using the solder alloy, and a method of manufacturing the electronic board.

According to one embodiment of the present invention, there is provided a solder alloy for flow soldering including 3.0 wt % to 14.0 wt % Zn, 0.003 wt % to 0.05 wt % Al, and the balance of Sn.

According to one aspect of the present invention, a solder alloy for flow soldering having a melting point lower than that of the conventional Sn—Ag—Cu-based solder alloy, a good solder wicking characteristic, and good solderability is provided. Further, according to one aspect of the present invention, so-called Cu erosion, a phenomenon where a solder alloy erodes a land formed of Cu, is prevented in the case of heating a soldered joint for a long period of time, for example, several tens of seconds, at the time of rework.

According to one embodiment of the present invention, there is provided an electronic board including an electronic component and a circuit board, wherein the electronic component and the circuit board have a soldered joint formed of the solder alloy as set forth above.

According to one embodiment of the present invention, there is provided a method of manufacturing an electronic board, including a step of attaching an electronic component to a circuit board and a joining step of melting a solder alloy formed of a Sn—Zn—Al alloy and soldering the electronic component to the circuit board by bringing the molten solder alloy into contact with the circuit board.

According to one aspect of the present invention, it is possible to achieve flow soldering with good solderability by significantly suppressing generation of Zn oxide dross by adding Al to a Sn—Zn solder alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a flow soldering apparatus according to one embodiment of the present invention;

FIG. 2 is a schematic diagram showing a flow solder pot of the flow soldering apparatus of FIG. 1;

FIG. 3A is a diagram for illustrating a method of evaluating a solder wicking characteristic;

FIG. 3B is another diagram for illustrating the method of evaluating a solder wicking characteristic;

FIG. 4 and FIG. 5 are tables showing the characteristic test results of a Sn—Zn—Al solder alloy;

FIG. 6 is a graph showing the results of testing the Cu erosion characteristic of the Sn—Zn—Al solder alloy;

FIG. 7A is a diagram showing a process of an electronic board manufacturing method according to one embodiment of the present invention;

FIG. 7B is a diagram showing another process of the electronic board manufacturing method; and

FIG. 8 is a diagram for illustrating a method of managing the composition of molten solder in the flow soldering apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given, with reference to the accompanying drawings, of one or more embodiments of the present invention.

According to one embodiment of the present invention, there is provided a solder alloy used for a soldering method in which the solder alloy is melted and the solder joint of an electronic component and an electronic board is soldered by bringing the molten solder alloy into contact with the solder joint, such as flow soldering. Flow soldering includes flow soldering in the narrow sense, which forms a flow of molten solder and brings the flow into contact with a solder joint, and dip soldering, which dips a solder joint in molten solder. Here, the case of using flow soldering in the narrow sense is shown.

FIG. 1 is a schematic diagram showing a flow soldering apparatus 10 according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing a flow solder pot (bath) 20 of the flow soldering apparatus 10.

Referring to FIGS. 1 and 2, the flow soldering apparatus 10 includes a spray fluxer 11 that applies flux to a printed circuit board 14, a first heater 12, a second heater 13, the flow solder pot 20, and a conveyor 15 that conveys the printed circuit board 14. The first heater 12, the second heater 13, and the flow solder pot 20 are covered by an enclosure 16. The printed circuit board 14 loaded with electronic components is conveyed from left to right on the plane of the paper by the conveyor 15. The printed circuit board 14 is cooled by a cooler (not graphically illustrated) after being subjected to solder joining in the flow solder pot 20.

The spray fluxer 11 includes a flux tank 17 and a jet mechanism 19 having a nozzle 18. The spray fluxer 11 atomizes flux supplied from the flux vessel 17 with the nozzle 18, and sprays and applies the atomized flux to the lower surface of the printed circuit board 14.

Each of the first heater 12 and the second heater 13 includes an electrothermal heater. The first heater 12 performs preliminary heating on the lower surface of the printed circuit board 14. The second heater 13 further heats the lower surface of the printed circuit board 14 to a predetermined temperature, that is, substantially the same temperature as the molten solder in the flow solder pot 20.

The flow solder pot 20 includes a solder pot (bath) 21, a heater that heats solder and a temperature control part (neither of which is graphically illustrated), a first nozzle vessel 22, a second nozzle vessel 23, impellers 24 for spouting molten solder into the first nozzle vessel 22 and the second nozzle vessel 23, impeller shafts 25 that transmit the rotations of the corresponding impellers 24, and rotational driving parts 26 having respective motors. A primary nozzle 28 and a secondary nozzle 29 forming a double-wave system are provided on top of the first nozzle vessel 22 and the second nozzle vessel 23, respectively.

The double-wave system is suitable for soldering chip-type electronic components and components with lead legs to a printed circuit board. The primary nozzle 28 ejects molten solder upward and moves back and forth in the directions perpendicular to the moving direction of the printed circuit board 14, thereby soldering chip-type electronic components (not graphically illustrated) to the printed circuit board 14 while removing flux therebetween. Further, the secondary nozzle 29 has a liquid receiver part 29 a so that the secondary nozzle 29 ejects molten solder so as to form a flat liquid surface in the moving direction of the printed circuit board 14. The secondary nozzle 29 mainly solders electronic components with lead legs (not graphically illustrated). Although not graphically illustrated, the flow solder pot 20 may also be of a wave system omitting the primary nozzle 28 and formed only of the secondary nozzle 29, a flow dip system, or a single flow system.

In the flow soldering apparatus 10, molten solder is pushed upward into a flow by the rotation of each impeller 24 so as to come into contact with the printed circuit board 14, so that soldering is performed. The solder not consumed by soldering moves downward in the solder pot 21 and flows into the primary nozzle vessel 22 and the secondary nozzle vessel 23 to become another flow. Thus, the molten solder constantly circulates. Accordingly, there is little variation in temperature or composition, so that it is easy to obtain good solderability.

According to one embodiment of the present invention, there is provided a Sn—Zn—Al solder alloy used in such a flow soldering apparatus. The Sn—Zn—Al alloy is lead-free solder. As shown in the table below, the Sn—Zn—Al alloy has a melting point of 201° C. to 224° C. around compositions of the present invention. This melting point depends only on the composition ratio of Sn and Zn, and is constant in the Al content range of the present invention without depending on the Al content.

The inventor of the present invention has found that a Sn—Zn—Al solder alloy for flow soldering according to one embodiment of the present invention also has good solderability in a composition range extended to the side of a greater Al content than the Sn—Zn—Al-based solder alloy of Japanese Patent No. 3357045 described in the BACKGROUND section of this specification, which is used principally as solder paste. That is, according to the composition range of a Sn—Zn—Al solder alloy of one embodiment of the present invention, Zn is more than or equal to 3.0 wt % and less than or equal to 14.0 wt % (3.0 wt % to 14.0 wt %), Al is more than or equal to 0.0030 wt % and less than or equal to 0.050 wt % (0.0030 wt % to 0.050 wt %), and the balance is Sn. According to this Sn—Zn—Al solder alloy, its Al component itself is oxidized to form a coat, thereby preventing oxidation of Zn. In the flow soldering apparatus, when molten solder is ejected from a nozzle, the Al oxide film is broken by the force of the ejected molten solder, so that non-oxidized molten solder comes into contact with a printed circuit board. Therefore, it is believed that in flow soldering, the Al oxide film is prevented from impairing solderability so that good solderability can be obtained with a composition having a greater Al content than in the case of solder paste.

TABLE COMPOSITION (wt %) MELTING Zn Al Sn POINT (° C.) 2.0 0.006 BALANCE 224.4 3.0 0.006 BALANCE 221.0 7.0 0.006 BALANCE 201.6 9.0 0.006 BALANCE 201.9 14.0 0.006 BALANCE 202.5

The inventor of the present invention tested the composition range of the Sn—Zn—Al solder alloy used for flow soldering by evaluating its through hole solder wicking characteristic and Cu erosion characteristic.

[Evaluation of Solder Wicking Characteristic]

The inventor of the present invention evaluated the through hole solder wicking characteristic in the atmosphere with respect to various compositions of the Sn—Zn—Al solder alloy.

FIG. 3A and FIG. 3B are diagrams for illustrating a method of evaluating the solder wicking characteristic. FIG. 3A is a cross-sectional view of a printed circuit board 30 passing through the center of a through hole 31. FIG. 3B is a plan view of the upper surface of the printed circuit board 30. In FIG. 3B, a graphical illustration of an electronic component 33 shown in FIG. 3A is omitted.

Referring to FIG. 3A and FIG. 3B, first, the through hole 31 was provided in the printed circuit board 30, and a land 32 a and a land 32 b were provided on the upper surface and the lower surface, respectively, of the printed circuit board 30. Solder joining was performed with a lead leg 34 of the electronic component 33 being inserted in the through hole 31. The through hole 31 is formed of a Cu material, and has an inside diameter of 0.8 mm and a length L0 of 1.6 mm. The electronic component 33 is a 40-pin type, and is subjected to either Sn plating or Ni—Au plating as surface processing on its lead legs 34. Five electronic components 33 of Sn plating and five electronic components 33 of Ni—Au plating were used. That is, 400 soldered joints were formed per sample. Each of the lead legs 34 of each electronic component 33 used has a rectangular cross-sectional shape of 0.5 mm×0.5 mm.

Solder joining was performed using a small-size flow solder pot (TOP-324A of Techno Design Industry Co., Ltd.). The lower surface of the printed circuit board 30, on which flux (HSX03 of Senju Metal Industry Co., Ltd.) was applied, was brought into contact with a flow of solder for approximately 5 minutes using a bath of the molten solder of each of Samples No. 1 through No. 60 shown in FIG. 4 and FIG. 5.

The solder wicking characteristic was evaluated by observing a cross section of the through hole 31 with a metallurgical microscope, measuring a length L1 of a solidified solder 35 between its upper surface and the land 32 b (or the distance between the upper surface of the solidified solder 35 and the land 32 b), and determining the filling rate (%) from the ratio of L1 to the length L0 of the through hole 31 (=L1/L0×100) as shown in FIG. 3A. A greater filling rate is more preferable.

Further, the land wettability was determined by visually checking the angle (angular range) θ of the solidified region of the land 32 a around the lead leg 34 on the upper surface of the printed circuit board 30. A larger angle θ indicates better land wettability. Further, all of the 400 soldered joints were evaluated in each sample.

FIG. 4 and FIG. 5 are tables showing the characteristic test results of the Sn—Zn—Al solder alloy. FIG. 4 and FIG. 5 show the composition ratios and the solder wicking characteristics of solder-alloy Sample Nos. 1 through 60 formed of 2.0 wt % to 19.0 wt % (more than or equal to 2.0 wt % and less than or equal to 19.0 wt %) Zn, 0.000 wt % to 0.070 wt % (more than or equal to 0.000 wt % and less than or equal to 0.070 wt %) Al, and the balance is Sn. Further, in the tables, “double circle” indicates that all of the evaluated 400 soldered joints have a filling rate of 75% or more and a land wettability of 180° or more, “single circle” indicates that all of the evaluated 400 soldered joints have a filling rate of 75% or more and a land wettability of 0° to less than 180°, and “cross” indicates that at least one of the evaluated 400 soldered joints has a filling rate of less than 75%. Those indicated by “double circle” or “single circle” are acceptable, and those indicated by “cross” are unacceptable.

[Effect of Zn Content]

According to FIG. 4 and FIG. 5, where the Zn content is 3.0 wt % to 14.0 wt %, the samples of compositions in which the Al content is 0.003 wt % to 0.050 wt % (Sample Nos. 12 to 17, 22 to 26, 32 to 36, and 42 to 46) have a good solder wicking characteristic as indicated by “double circle” or “single circle.” In particular, the samples of compositions in which Zn is 7.0 wt % to 11.0 wt %, the Al content is 0.003 wt % to 0.030 wt %, and the balance is Sn (Sample Nos. 13 to 15, 23 to 25, and 33 to 35) and the sample having a composition in which Zn is 7.0 wt % to 9.0 wt %, the Al content is more than 0.030 wt % and less than or equal to 0.050 wt %, and the balance is Sn (Sample Nos. 43 and 44) have a very good solder wicking characteristic as indicated by “double circle.”

Further, FIG. 4 shows that the solder wicking characteristic is also good in the case of Sample No. 21, where the Zn content is more than or equal to 2.0 wt % and less than 3.0 wt % in composition.

[Effect of Al Content]

According to FIG. 4 and FIG. 5, where the Al content is 0.003 wt % to 0.050 wt %, the samples of compositions in which the Zn content is 3.0 wt % to 14.0 wt % (Sample Nos. 12 to 17, 22 to 26, 32 to 36, and 42 to 46) have a good solder wicking characteristic as indicated by “double circle” or “single circle.” In particular, within the range of the above-described compositions, the samples of compositions in which the Al content is more than 0.008 wt % and less than or equal to 0.050 wt % (Sample Nos. 22 to 26, 32 to 36, and 42 to 46) have a good solder wicking characteristic compared with the Sn—Zn—Al solder alloy of Japanese Patent No. 3357045 described in the BACKGROUND section of this specification.

Further, it is also shown that where the Al content is more than 0.050 wt % and less than or equal to 0.070 wt %, the samples of compositions in which the Zn content is 7.0 wt % to 9.0 wt % (Sample Nos. 53 and 54) have a good solder wicking characteristic as indicated by “single circle.”

Further, it is also shown that even in the case where the Al content is 0.000 wt %, the samples of compositions where the Zn content is 7.0 wt % and 9.0 wt % (Sample Nos. 3 and 4) have a good solder wicking characteristic as indicated by “single circle.” However, this is not practical because the Cu erosion characteristic described below is degraded if the Al content is 0.000 wt %.

[Evaluation of Cu Erosion Characteristic]

The inventor of the present invention studied the relationship between the Al content and the Cu erosion characteristic of the Sn—Zn—Al solder alloy. The Cu erosion characteristic represents the characteristic of alloying and eroding a Cu material when a molten solder alloy comes into contact with the Cu material of lands or through holes. The thickness of the eroded region of the Cu material is expressed as the amount of Cu erosion. That is, a conspicuously high Cu erosion characteristic results in chipped or lost lands. In particular, the occurrence of Cu erosion is conspicuous at the time of heating a solder alloy at temperatures higher than or equal to its melting point for several tens of seconds as in rework.

The Cu erosion characteristic was evaluated by immersing a Cu plate material in a solder alloy of a Sn-9 wt % Zn—Al composition and determining a change in the plate thickness by measuring the plate thicknesses before and after the immersion. Solder alloys with different Al contents in composition, varying from 0.000 wt % to 0.070 wt %, were prepared with the Sn content being the remaining content in composition. Further, the temperature of the molten solder alloy was 260° C., and the Cu plate material was immersed with two different conditions, that is, for 20 seconds and for 40 seconds. As a comparative example, the Cu erosion characteristic of Sn-3 wt % Ag-0.5 wt % Cu was also evaluated.

FIG. 6 is a graph showing the results of testing the Cu erosion characteristic of the Sn—Zn—Al solder alloy. In FIG. 6, the amount of Cu erosion of Sn-9 wt % Zn—Al is shown for the case of an immersion time of 20 seconds (as indicated by “circle”) and for the case of an immersion time of 40 seconds (as indicated by “square”).

Referring to FIG. 6, the amount of Cu erosion is maximized at an Al content of 0.000 wt %, and sharply decreases as the Al content increases, so as to be substantially constant or slightly decreasing where the Al content is more than or equal to 0.010 wt %. For comparison, in the case of using Sn-3 wt % Ag-0.5 wt % Cu as the solder alloy, the amount of Cu erosion is 15 μm in the case of immersion for 20 seconds and 40 μm in the case of immersion for 40 seconds. Letting the amount of Cu erosion less than that of Sn-3 wt % Ag-0.5 wt % Cu be good, the amount of Cu erosion is good where the Al content is 0.003 wt % or more in each of the case of 20 second immersion and the case of 40 second immersion.

From the above-described evaluations, it has been determined that solder alloys where Zn is more than or equal to 3.0 wt % and less than or equal to 14.0 wt %, Al is more than or equal to 0.0030 wt % and less than or equal to 0.050 wt %, and the balance is Sn have good solder wicking and Cu erosion characteristics. Further, it has been determined that solder alloys of compositions where Zn is more than or equal to 7.0 wt % and less than or equal to 9.0 wt %, Al is more than or equal to 0.050 wt % and less than or equal to 0.070 wt %, and the balance is Sn and solder alloys of compositions where Zn is more than or equal to 2.0 wt % and less than or equal to 3.0 wt %, Al is 0.010 wt %, and the balance is Sn also have good solder wicking and Cu erosion characteristics.

Next, a description is given of a method of manufacturing an electronic board using the above-described solder alloy according to one embodiment of the present invention.

FIG. 7A and FIG. 7B are diagrams showing processes of the electronic board manufacturing method of this embodiment.

In the process of FIG. 7A, the electronic component 33 and an electronic component 36 are attached to the printed circuit board 30. Specifically, the lead legs of the electronic component 33 are inserted into the corresponding through holes 31. Further, the chip-type electronic component 36 is fixed to the printed circuit board 30 with an adhesive agent 39.

In the process of FIG. 7B, the electronic components 33 and 36 are soldered by flow soldering. Specifically, soldering is performed with the flow soldering apparatus 10 shown in FIG. 1 and FIG. 2 using a Sn—Zn—Al solder alloy according to one embodiment of the present invention. The solder pot 21 shown in FIG. 2 is filled with a molten Sn—Zn—Al solder alloy of a predetermined composition. A flow of the Sn—Zn—Al solder alloy is formed with double-wave system nozzles so as to come into contact with the lower surface (the side on which the electronic component 36 is attached) of the printed circuit board 30 to which flux and heat have been applied. As a result, as shown in FIG. 7B, the lead legs 34 of the electronic component 33 and electrodes 37 of the electronic component 36 are soldered to the corresponding through holes 31 and corresponding lands 38, respectively.

At this point, the Sn—Zn—Al solder alloy having a composition where Zn is more than or equal to 3.0 wt % and less than or equal to 14.0 wt % has a melting point of 199° C. to 222° C., and considering temperature unevenness in the printed circuit board 30, it is preferable that the Sn—Zn—Al solder alloy be heated to temperatures 30° C. to 50° C. higher than its melting point.

In the process of FIG. 7B, the molten solder in the flow solder pot 20 (the solder pot 21) of the flow soldering apparatus 10 is managed as follows.

FIG. 8 is a diagram for illustrating a method of managing the composition of molten solder in the flow soldering apparatus 10. Referring to FIG. 8, for example, the flow solder pot 20 of the flow soldering apparatus 10 is filled with a solder alloy of 300 kg. Assuming that the solder alloy has a composition of Sn-9 wt % Zn-0.010 wt % Al at the start of the operation, the amount of Al contained in the molten solder is 30 g. As described above, good solderability is shown when the Al content is 0.003 wt % to 0.050 wt %. The Al content of this range corresponds to 9 g to 90 g of Al in amount. Accordingly, the composition of the molten solder is managed so that the amount of Al is always within this range, that is, between 20 g to 70 g.

Therefore, Al material is supplied in accordance with the amount of Al oxidized into dross and lost. When the temperature of the molten solder alloy is 260° C., the amount of Al oxidized into dross in the molten solder alloy is 1% of Al per hour. That is, 1% of 30 g, i.e., 0.3 g is converted into dross per hour.

For example, it is assumed that a solder alloy having a composition of Sn-9 wt % Zn-0.010 wt % Al (Al amount of 30 g) is replenished with 60 g of Al material at the start of the operation so that the initial amount of Al is 90 g. In this case, the time required for the initial amount to be reduced to 20 g is (90-20)/0.3=233 hours. That is, the amount of Al converted into dross may be supplemented within 233 hours. FIG. 8 shows the case of supplying a 50 g tablet of Al material by estimating how much time elapses before the amount of Al becomes 20 g. Supplying 50 g of Al material makes the amount of Al contained in the molten solder 70 g, so that the amount of Al can be managed within the management range.

According to this method of managing the composition of a molten solder alloy, the composition of a Sn—Zn—Al solder alloy is maintained within a good solderability range by supplying Al material in accordance with the amount of Al oxidized into dross in the Sn—Zn—Al solder alloy.

The Al material used is a tablet or ingot of pure solid Al material. When solid Al material is introduced into the molten Sn—Zn—Al solder alloy, the solid Al material is easily dissolved in the molten solder alloy. Further, the Al material to be supplied may be a Zn—Al alloy, which may further include Sn. Examples of such material include solid or molten Zn-5 wt % Al (having a melting point of 380° C.)

Since the temperature of molten solder can be lower than that of the Sn—Ag—Cu-based solder alloy, the printed circuit board 30 can be reduced in thermal stress applied to the electronic components 33 and 36. Further, the printed circuit board 30 has excellent reliability because of a good through hole wicking characteristic and wettability.

In a manufacturing method according to one embodiment of the present invention, flow soldering with good solderability can be achieved by significantly suppressing generation of Zn oxide dross by adding Al to a Sn—Zn solder alloy.

Further, in a manufacturing method according to one embodiment of the present invention, of a molten solder alloy, Al is selectively oxidized and consumed. Therefore, good solderability can be retained by supplying a material containing Al in accordance with the amount of oxidized Al. Further, the composition range of the solder alloy can be easily managed by estimating the oxidation rate of Al and replenishing the material containing Al at predetermined intervals.

Further, by adding Al to a Sn—Zn solder alloy, it is possible to prevent the stainless steel of the solder pot of a flow soldering apparatus from being eroded. This is because Al is interposed between the surface of the stainless steel and a bath of molten solder so as to prevent Sn from eroding the stainless steel.

According to one embodiment of the present invention, there is provided a solder alloy for flow soldering including 3.0 wt % to 14.0 wt % Zn, 0.003 wt % to 0.05 wt % Al, and the balance of Sn.

According to one aspect of the present invention, a solder alloy for flow soldering having a melting point lower than that of the conventional Sn—Ag—Cu-based solder alloy, a good solder wicking characteristic, and good solderability is provided. Further, according to one aspect of the present invention, so-called Cu erosion, a phenomenon where a solder alloy erodes a land formed of Cu, is prevented in the case of heating a soldered joint for a long period of time, for example, several tens of seconds, at the time of rework.

According to one embodiment of the present invention, there is provided an electronic board including an electronic component and a circuit board, wherein the electronic component and the circuit board have a soldered joint formed of the solder alloy as set forth above.

According to one embodiment of the present invention, there is provided a method of manufacturing an electronic board, including the step of attaching an electronic component to a circuit board and the joining step of melting a solder alloy formed of a Sn—Zn—Al alloy and soldering the electronic component to the circuit board by brining the molten solder alloy into contact with the circuit board.

According to one aspect of the present invention, it is possible to achieve flow soldering with good solderability by significantly suppressing generation of Zn oxide dross by adding Al to a Sn—Zn solder alloy.

Additionally, the joining step may supply the molten solder alloy with a material containing Al in accordance with the amount of oxidized Al in the molten solder alloy. Since Al is selectively oxidized and consumed in the molten solder alloy, the solder alloy can be managed to be within a predetermined composition range of good solderability by supplying a material containing Al in accordance with the amount of oxidized Al.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 

1. A solder alloy for flow soldering, comprising: 3.0 wt % to 14.0 wt % Zn; 0.003 wt % to 0.05 wt % Al; and the balance of Sn.
 2. The solder alloy for flow soldering as claimed in claim 1, wherein Al is more than 0.008 wt %.
 3. The solder alloy for flow soldering as claimed in claim 1, wherein Zn is 7.0 wt % to 11.0 wt %.
 4. The solder alloy for flow soldering as claimed in claim 3, wherein: Zn is less than or equal to 9.0 wt %; and Al is more than 0.003 wt %.
 5. An electronic board, comprising: an electronic component; and a circuit board, wherein the electronic component and the circuit board have a soldered joint formed of the solder alloy as set forth in claim
 1. 6. A method of manufacturing an electronic board, comprising: a step of attaching an electronic component to a circuit board; and a joining step of melting a solder alloy formed of a Sn—Zn—Al alloy and soldering the electronic component to the circuit board by brining the molten solder alloy into contact with the circuit board.
 7. The method as claimed in claim 6, wherein the joining step supplies the molten solder alloy with a material containing Al in accordance with an amount of oxidized Al in the molten solder alloy.
 8. The method as claimed in claim 7, wherein the material containing Al is a solid Al material.
 9. The method as claimed in claim 7, wherein the material containing Al is one of a solid Zn—Al alloy and a molten Zn—Al alloy.
 10. The method as claimed in claim 6, wherein the solder alloy comprises: 3.0 wt % to 14.0 wt % Zn; 0.003 wt % to 0.05 wt % Al; and the balance of Sn. 